1. Field of the Invention
This invention relates to apparatus for producing single crystals and, more particularly, to apparatus employing the Liquid Encapsulated Czochralski method for producing compound semiconductor single crystals.
2. Description of the Prior Art
The Liquid Encapsulated Czochralski (LEC) method has been used for growing compound semiconductor single crystals, such as gallium phosphate (GaP), gallium arsenide (GaAs) and Indium phosphate (InP), from melt GaP, melt GaAs or melt Inp. In this method, the single crystal is grown at the surface of melt compound semiconductor material which is pressurized by inert gas, such as nitrogen (N2), and whose surface is covered by melt boron oxide (B.sub.2 O.sub.3) in order to prevent decomposition and vaporization of the compound semiconductor melt. In FIG. 1 showing a cross-sectional view of a conventional apparatus for growing the single crystals used for the LEC method, a crucible 1 is protected by a support wall 2. A compound semiconductor melt 3 having high specific gravity resides in the crucible 1, and melt boron oxide (B.sub.2 O.sub.3) 4 completely covers the surface of the compound semiconductor. A heater 5 melts the compound semiconductor melt material 3 and the (B.sub.2 O.sub.3) 4 and keeps these two compounds in a molten condition. The heater 5 is surrounded by a heat shield 6 in order to improve the efficiency of the heater in directing heat to the crucible 1. A crucible driving shaft 7 rotates the crucible 1 to mix the compound semiconductor melt and to equalize its temperature within the crucible 1. A thermocouple 8, which is embedded in the shaft 7, measures the temperature of the compound semiconductor melt.
A single crystal 9 is grown by gradually pulling it from the semiconductor melt. To this end, a pulling shaft 11 with a seed crystal 10 at one end is lowered through melt (B.sub.2 O.sub.3) 4 into contact with the surface of the compound semiconductor melt 3, to allow growth to begin, and is then gradually raised. The shaft 11 is connected to rotating and reciprocating apparatus (not shown) which drives the shaft at predetermined pulling and rotation rates. All of the elements shown in FIG. 1 are contained in a high-pressure housing 12 filled with N.sub.2 gas at a pressure higher than that of the equilibrium dissociation pressure of the compound semiconductor melt 3.
It has been found that the conventional growing apparatus described above cannot completely prevent the decomposition and vaporization of the compound semiconductor melt because decomposition and vaporization of the compound semiconductor melt occurs during the melting phase of the compound semiconductor material in the growth chamber before crystal growing begins. Because polycrystalline particles with various shapes are used as crude material for the compound semiconductor melt, the particles fill the crucible 1 with interstices or spaces between them. The total volume of particles occupying the crucible 1 is about 60 percent of the volume of the crucible at best. As an example, the ratio of weight to volume of GaP polycrystalline particles filling the crucible is less than 2.5 gr/cc; however, the specific gravity of GaP itself is 4.13 gr/cc.
When the crucible 1, which contains the solid particles of compound semiconductor material and the solid B.sub.2 O.sub.3, is heated, the solid B.sub.2 O.sub.3 melts at about 600.degree. C. The semiconductor particles do not melt at this temperature. The melting temperature of the particles is higher and depends on the particular material. For example, the melting temperature of GaP, GaAs and InP is 1470.degree. C., 1237.degree. C., and 1062.degree. C., respectively. During heating of the crucible, both the melt B.sub.2 O.sub.3 and the particles of compound semiconductor exist in the crucible at 600.degree. C., which is the melting temperature of B.sub.2 O.sub.3, to more than 1000.degree. C.
Because of these differences in melting point, the conventional apparatus for growing single crystals cannot prevent decomposition and vaporization of the compound semiconductor. For example, if an amount of B.sub.2 O.sub.3 such as a layer of melt B.sub.2 O.sub.3, is used, which does not completely cover the surface of the compound semiconductor particles, the decomposition and vaporization of the compound semiconductor are not prevented when the semiconductor particles begin to melt. On the other hand, if an amount of melt B.sub.2 O.sub.3 is used which completely covers the particles of the compound semiconductor, the layer of the melt B.sub.2 O.sub.3 may be so thick that it causes deterioration of the temperature gradient at the surface of the melt compound semiconductor so that compound semiconductor single crystals can not grow.
This problem becomes more serious when compound semiconductors are used to grow single crystals with large diameters.